1. Field of the Invention
This invention relates to the separation of oil well fluids. In one aspect it relates to a three-phase separator.
2. Description of the Prior Art
Produced fluids from oil wells almost always include oil and gas and frequently include water. Surface facilities for handling such fluids generally include separators for isolating and segregating the three fluids. The three-phase separator is one type of separator used in oil and gas separation and offers the advantage that the complete fluid stream from the well may be handled by one vessel. If the separator functions properly, the oil and water and gas are separated, metered, and directed to their respective handling facilities. In the three-phase separator, the oil and water are normally collected in separate compartments and discharged through separate outlets to the handling and measuring facilities, and the gas is discharged into the gas gathering system. The three-phase separator may be employed to handle the total production from a particular lease or group of wells (i.e. production separator); or may be used to treat only a single well to determine the volume and fluid characteristics of the well (i.e. test separator).
The three-phase separator is particularly suited for test well applications since it permits metering each of the three streams discharging from the separator. The results of well tests are used to allocate production to a particular well. This data in turn is used as a basis for royalty payments, to determine performance of the reservoir and producing equipment and as a guide for implementing conservation measures to attain optimum production. In many states, well test data are used by regulatory agencies to set production allowables. It will be appreciated that the accuracy of well tests is of vital importance.
Test and production separators must be capable of handling a variety of producing conditions including large differences in flow rates, fluid densities, and relative amounts of oil, gas and water. All of these factors affect operation and accuracy of the three-phase separator. The problem of accuracy and operation will best be understood when considering the construction of a preferred type of three-phase separator. These separators are normally divided into four compartments, which will be referred to herein as the emulsion compartment, oil compartment, free-water compartment and water collecting compartment. A fixed bulkhead, referred to as the oil weir, separates the emulsion compartment and the oil compartment, and an adjustable bulkhead referred to as the water weir separates the free water compartment and the water collecting compartment.
Produced fluids from the well enter the separator at the emulsion compartment where gas separates from the liquid and is directed to the gas discharge valve. Free oil collects at the top of the emulsion compartment and free water gravitates to the bottom of that compartment. A horizontal transition zone, usually in the form of emulsion, separates the two free liquids in the emulsion compartment. The free oil spills over the fixed weir into the oil compartment while free water passes under the oil compartment into the free water compartment, and from there it spills over the water weir into the free water collecting compartment. It is thus seen that the relative elevations of the oil weir and the water weir must be properly adjusted to provide proper separation. If the water weir is too low in relation to the oil weir for a set of producing conditions, the emulsion can pass into the free water compartment and be discharged along with the free water. This not only affects the accuracy of the measurements but results in the loss of oil and fouls the water handling facilities. On the other hand, if the water weir is too high in relation to the oil weir, emulsion retention time within the emulsion compartment will be shortened and emulsion can enter the oil compartment.
For a given set of producing conditions, the proper relative elevations of the two weirs can be achieved with presented available separators. However, the producing conditions frequently are not known but are merely predicted. Moreover, such conditions change from day to day making it practically impossible for the operator to make the necessary adjustments with the presently available three-phase separators. If water is detected in the free oil or if oil is detected in the free water, the operator normally tries to correct these conditions by adding more emulsifying chemical. This results in an inefficient and expensive use of chemicals.